Bis(cyclopentadienyl) complexes of the general formula Cp'.sub.2 MR.sub.2 wherein Cp' is a substituted or unsubstituted cyclopentadienyl radical, M is Ti, Zr or Hf, and R is alkyl, are well known in the art. These complexes can be employed together with alkyl aluminum and alumoxane cocatalysts to produce active olefin polymerization catalysts. Particularly active catalysts are produced when these complexes are employed together with alumoxane cocatalysts. W. Kaminsky et al., "Polymerization Of Olefins With A Homogeneous Zirconium/Alumoxane Catalyst", Adv. Polyolefins [Proc. ACS Int. Symp.], Meeting Date 1985, 361-371. Edited by: Seymour, Raymond B.; Cheng, Tai. Plenum: New York, N.Y., published 1987.
It is postulated that the mechanism by which these catalyst systems polymerize olefins involves the formation of cationic d.degree. metallocene alkyl complexes of the formula (Cp'.sub.2 MR).sup.+ which are ion paired with a negatively charged aluminumate counterion to form an active ionic catalyst system.
Bis(cyclopentadienyl) complexes having the formula Cp'.sub.2 MX.sub.2 wherein Cp' is as above defined and X is halogen are also well known in the art. These complexes can also be employed with alryl aluminum and alumoxane cocatalysts to produce active olefin polymerization catalysts. The activity of these catalysts is likewise attributed to formation of cationic d.degree. metallocene alkyl complexes of the formula (Cp'.sub.2 MR).sup.+ as intermediates which then coordinate with a negatively charged counterion. In this instance the intermediate product is produced by alkylation with the aluminum cocatalyst. This catalyst system is also ionic in nature.
In both instances, the polymerization activity of the catalyst system is attributed to formation of the highly coordinatively and electronically unsaturated 14-electron (Cp'.sub.2 MR).sup.+ cation which is capable of coordinating and inserting olefins into the M-R bond. Also in both instances, a labile stabilizing anion is required to stabilize the active cation species. In addition, these systems require an undesirable excess of aluminum cocatalyst to function effectively, particularly when an alumoxane is employed as cocatalyst.
Richard F. Jordan, "Cationic Metal-Alkyl Olefin Polymerization Catalysts", Journal of Chemical Education, April 1988, Vol. 65, No. 4, 285-289, discloses the preparation of zirconocene complexes such as [Cp.sub.2 Zr(CH.sub.3)(THF)].sup.+ [BPh.sub.4 ].sup.- wherein Cp is C.sub.5 H.sub.5, THF is C.sub.4 H.sub.8 O, and Ph is C.sub.6 H.sub.5.
The discovery of these complexes made it possible to employ a metallocene complex to polymerize ethylene in the absence of an aluminum cocatalyst. However, such complexes were found to polymerize ethylene rather slowly due to the presence of THF.
Gregory G. Hlatky et al., "Ionic, Base-Free Zirconocene Catalysts for Ethylene Polymerization", J. Am. Chem. Soc., 1989, 111, 2728-2729, disclose that peralkylated zirconocenes such as Cp.sup.*.sub.2 ZrMe.sub.2, wherein Cp.sup.* is C.sub.5 Me.sub.5 and Me is a methyl radical, react with the diprotic carborane acid nido-C.sub.2 B.sub.9 H.sub.13 in pentane to form ionic monomethyl complexes such as Cp.sup.*.sub.2 ZrMe(C.sub.2 B.sub.9 H.sub.12). The large size and chemical inertness of the high-nuclearity polyhedral carborane anion (C.sub.2 B.sub.9 H.sub.12).sup.- is said to make it a compatible counterion for the cationic dicyclopentadienyl alkyl complex (Cp.sup.*.sub.2 ZrMe).sup.+. Such complexes are also said to polymerize ethylene in the absence of an aluminum cocatalyst and to be highly active compared to the Jordan complexes.
M. Frederick Hawthorne, "The Chemistry of the Polyhedral Species Derived from Transition Metals and Carboranes", Acc. Chem. Res., 1968, 1, 281 has disclosed the "sandwich" bonding of a cyclopentadienyl ion (C.sub.5 H.sub.5).sup.- and a (3)-1,2-dicarbollide ion [(3)-1,2-B.sub.9 C.sub.2 H.sub.11 ].sup.2- to both iron and cobalt to produce compounds of the type [.pi.-C.sub.5 H.sub.5 ]Fe[.pi.-(3)-1,2-B.sub.9 C.sub.2 H.sub.11 ]. However, these compounds have not been described as useful polymerization catalysts.